1. Field of the Invention
The present invention relates to a method for preparing a non-aqueous dispersion of particles of a metal and/or a metal compound. More particularly, the present invention is concerned with a method for preparing a non-aqueous dispersion of particles of a metal and/or a metal compound from an aqueous dispersion of the same particles, which comprises contacting an aqueous dispersion of particles of a metal and/or a metal compound with a water-immiscible, non-aqueous liquid in the presence of a surfactant and in the presence or absence of a water-soluble inorganic acid salt and/or a water-soluble organic acid salt exhibiting substantially no surface activity, wherein when the contacting of the aqueous dispersion with the non-aqueous liquid is conducted in the absence of the salt, the salt is added after the contacting, thereby causing the particles to be migrated from the aqueous dispersion into the non-aqueous liquid.
2. Discussion Of Related Art
With respect to the method for chemically preparing metallic particles and/or metal compound particles, a method has conventionally been known in which a metal salt as a starting material is dissolved in water and/or a water-miscible liquid and the resultant solution is then treated with a suitable agent, such as a reducing agent, a boride-forming agent, a sulfide-forming agent or a hydroxide-forming agent, to thereby prepare metallic particles and/or metal compound particles dispersed in the form of a dispersion thereof in the water and/or the water-miscible liquid.
Metallic particles and/or metal compound particles, in the form of a composite having these particles borne on an inorganic or organic solid carrier, are used as catalysts, electronic materials, magnetic recording materials, gas sensor materials and the like. Further, metallic particles and/or metal compound particles, in the form of a composite having these particles dispersed in an organic resin, are used as electrically conductive pastes and the like. However, in either case, when the metallic particles and/or metal compound particles contain water, various disadvantages are likely to be caused. It is also known that when metallic particles and/or metal compound particles containing water are used as a pigment for an inorganic frit, the color tone of the frit is disadvantageously deteriorated.
One of the conventional methods for preparing metallic particles and/or metal compound particles dispersed in a water-immiscible, non-aqueous liquid is the so-called in-gas vaporizing method. One example of this method comprises vaporizing a metal by heating in a helium atmosphere at 10 to 50 Torr, guiding the resultant metal vapor with the aid of argon gas, mixing the metal vapor with a non-aqueous liquid vapor in the course of the guidance of the metal vapor, condensing the resultant mixture in a cold trap of liquid nitrogen, and then fusing the resultant condensate by heating, thereby obtaining a dispersion of metallic particles. This method has disadvantages in that expensive facilities are required and that energy, such as electricity, is consumed in a large quantity. Further, with respect to this method, in N. Satoh and K. Kimura, "The Bulletin of Chemical Society of Japan," Vol. 62, pp.1758-1763 (1989), it is described that when n-hexane is used as a non-aqueous liquid, metallic particles are aggregated and precipitated with respect to all types of metals, thereby rendering it practically impossible to obtain a non-aqueous dispersion of the metallic particles. Further, when benzene is used as a non-aqueous liquid, metallic particles are aggregated and precipitated with respect to almost all types of metals, thereby rendering it practically impossible to obtain a non-aqueous dispersion of metallic particles.
Another method is known as a microemulsion method. In this method, an oil-soluble surfactant forms microemulsion particles of reversed micelles in a water-immiscible, non-aqueous liquid. Into the resultant microemulsion particles is inevitably incorporated a very small amount of water, and the resultant water-containing microemulsion particles are dispersed in a large quantity of the non-aqueous liquid. When the very small amount of water has a noble metal salt dissolved therein, addition of a reducing agent gives a dispersion of noble metallic particles and/or noble metal compound particles. In this method, however, the amount of water serving to dissolve the noble metal salt is too small to obtain a dispersion having a high particle concentration. Further, the water cannot be prevented from entering, as microemulsion particles of reversed micelles, into a non-aqueous liquid.
Further, the so-called metal salt-extraction method has recently been employed. In this method, a noble metal salt is migrated from an aqueous solution thereof into a non-aqueous liquid phase using an extracting agent, followed by reduction with a reducing agent added to the aqueous phase, to thereby prepare a non-aqueous dispersion of noble metallic particles. However, this method has disadvantages in that a large amount of an extracting agent capable of dissolving in a non-aqueous liquid is required, and that a large amount of the extracting agent is inevitably contained in the prepared non-aqueous dispersion of noble metallic particles. Separation of the extracting agent from the obtained dispersion is difficult.
As mentioned above, although a non-aqueous dispersion of metallic particles and/or metal compound particles has high utility, the preparation thereof has been difficult. Therefore, the development of a method for easily preparing a non-aqueous dispersion having metallic particles and/or metal compound particles uniformly dispersed therein has long been desired.